A non-defining relative clause is part of a sentence that provides extra information but is not essential to understanding its meaning and purpose. When used within a sentence, this is often surrounded on each end with a comma and typically begins with a relative pronoun such as "who" or "which." An example of a non-defining relative clause can be found in a sentence like "My friend, who lives in Japan, wrote me a letter about a new video game" as the clause "who lives in Japan." In this example, the clause provides additional information about the subject of the sentence, but the meaning and purpose would remain intact even without it.
The purpose of a non-defining relative clause within a sentence is to provide additional information about something within it. This information is not crucial to the sentence, however, and it does not impact the overall purpose in a meaningful way. They are referred to as "non-defining" or "non-identifying" because of this, while other relative clauses can provide this type of information.
A non-defining relative clause is usually separated from the rest of the sentence by a comma, which functions in much the same way as a bracket or parenthesis. For example, in the sentence "The mouse, which my mom bought me, is running in the wheel," the clause "which my mom bought me" is separated from the rest of the sentence by commas. This indicates that the information within the commas is not used to provide additional meaning. The non-defining relative clause in this case simply gives more information about the mouse that is the subject of the sentence.
These types of clauses usually begin with a relative pronoun such as "who" or "which," although "that" is not used for a non-defining relative clause. Such clauses can be identified within a sentence by looking for commas and a relative pronoun. In a sentence like "I ate pizza for lunch at my favorite restaurant, where I drank too much soda," the clause comes at the end of the sentence and does not have a comma after it. The structure and use of such a non-defining relative clause, however, remains the same.
When identifying a non-defining relative clause, it is important to ensure that it provides only additional information and not something crucial to the meaning of the sentence. In the sentence, "My sister, who is older than me, lives in Germany," the clause "who is older than me" is not used to indicate which sister. This means the speaker only has a single sister and is not using "who is older than me" to indicate between multiple sisters. The same phrase can be used in a sentence like "My sister who is older than me lives in Germany," in which it is now an integral part of its meaning. In this example, the speaker is using "who is older than" to identify one particular sister, providing additional meaning within the sentence, so it is no longer non-defining.